An Asp94Gly missense mutation in GP1BB causes severe Bernard-Soulier syndrome in an Iraqi family

D. Boeckelmann, E. Lerner, J. Schelling, K. Neubauer, K. Sandrock-Lang, B. Zieger (Freiburg, Germany)

Bleeding disorders, coagulation and fibrinolytic factors
Date: 17.02.2017,
Time: 17:15 - 18:15

Objective: Bernard-Soulier syndrome (BSS) is a rare, autosomal recessive bleeding disorder with abnormalities of the platelet GPIb-IX-V complex. Typical are mucocutaneous bleedings (easy bruising, epistaxis), thrombocytopenia, presence of giant platelets and impaired ristocetin- induced agglutination. Three disease-related genes (GP1BA, GP1BB, GP9) harbor mutations of all types. We investigated a family of middle east origin with two sisters presenting severe bleeding symptoms. Aim of the study was to define the phenotype and reveal it´s underlying pathogenic mutation.

Methods: Blood count, smear, platelet aggregometry (after stimulation with collagen, ristocetin, ADP, epinephrin) and flow cytometry analyses (CD41 (GPIIb/IIIa), GP42a (GPIX), CD42b (GPIb)) were performed. Sequencing of exons and splice site regions of all genes was done for index patient. Variants were analyzed by ALAMUT®. Occurrence in variant databases (dbSNP, EVS, ExAC), ClinVar, locus-specific mutation database of the BSS international consortium, conservation status and in silico pathogenic prediction (SIFT, MutTaster, PolyPhen2) were investigated. Genotyping was performed for all family members.

Results: Patients showed thrombocytopenia (11 and 7 G/l), giant platelets, impaired ristocetin-induced agglutination and severely reduced expression of GPIb/IX. Normal results were obtained for the other family members. Sequencing revealed a homozygous variant in exon 2 of GP1BB (coding for GPIb beta) in both patients (NM_000407.4: c.281A>G; p.Asp94Gly). This mutation was not listed in any of the above mentioned sequencing databases. Only the BSS consortium report mentioned this mutation, originally found in a Lebanese patient, without further information. Functional prediction is concordant pathogenic in all tools. The heterozygous parents are not affected and the brother shows wildtype sequence.

Conclusion: We report a mutation of the GP1BB gene leading to a clinical phenotype of BSS which so far was only mentioned in the BSS consortium report without any further information. The mutation is located in the C-terminal cysteine rich flanking region next to cysteine at p.93 participating in disulfide bonds. The exchanged amino acids differ in size and polarity and may influence the formation of disulfide bond next to them. This may cause an absent GPIb/IX expression which leads to the severe phenotype.